This application claims the benefit under 35 U.S.C. xc2xa7365 of French patent application No. 0114251 filed Oct. 30, 2001.
1. Field of the Invention
The invention relates to a curved waveguide element and to a transmission device comprising the said element.
2. Related Art
Transmission systems use high frequencies of the order of ten gigahertz or more. This is the case with high-rate radio systems such as, for example, transmissions by satellite where the frequency bands are in the region of 10 GHz or at higher frequencies. For these very high frequencies, it is known to use waveguide elements to receive the signals and to effect a first separation of these signals.
FIG. 1 shows the waveguide circuit of a transmitter/receiver device of a known type. The antenna here is a horn 1, for example placed facing a parabolic-type reflector which focuses the reflected waves into a waveguide 2, for example of square cross section. The waveguide 2 itself provides a high-pass filter function which selects the desired bandwidth. A power divider 3 divides the waveguide 2 into two guides of rectangular cross section on which two filters 4 and 5 are placed, these being intended to isolate, on the one hand, the reception frequency band and, on the other hand, the transmission frequency band. Placed at the open end of the filters 4 and 5 are electronic cards, for example produced in microstrip technology, which transpose the signals into an intermediate frequency band in order to transmit an electrical signal to a coaxial cable. To make it easier to produce the device, the electronic cards are placed in the same plane. The filter 4 is a high-pass filter produced simply with the aid of a change in waveguide cross section. The filter 5 is a low-pass filter, for example produced with irises.
Such a device is relatively bulky and requires the use of expensive materials in large quantity. This is because the filter 4 may be relatively long. The change in cross section may be made in several steps, each step having a length equal to at least one quarter of the wavelength associated with the cross section of the guide. In addition, on either side of a change in cross section, the waveguide must have a length equal to the wavelength associated with the cross section of the guide so as to obviate evanescent modes which may be excited in the discontinuities. Thus, the filter 4, although simple and effective, is generally longer than the filter 5, requiring the waveguide supporting the filter 5 to be extended.
The invention aims to reduce the size of the waveguide circuit. Contrary to the preconceptions of those skilled in the art for whom it is essential to maintain a constant waveguide cross section in the curved parts, the invention proposes to change the cross section in a curved part. Thus, the system consisting of the curved element and of the element having the change in cross section is reduced to the curved element.
The invention is an electromagnetic waveguide element comprising a first wave input/output along a first direction and a second wave input/output along a second direction, the first direction lying within a plane cutting the second direction, the first and second inputs/outputs being connected by at least one curved part. The curved part includes at least one curved portion of constant cross section bounded by two ends, at least one end corresponding to a change in cross section of the guide.
When the two ends of the portion correspond to a change in cross section of the guide, the curved length of the central axis of the waveguide of the portion is equal to an odd multiple of one quarter of the wavelength associated with the cross section of the guide of the portion.
According to a very compact embodiment, the curve of the central axis of the guide has at least one discontinuity at the said end of the portion which corresponds to a change in cross section.
Preferably, the said end corresponding to a change in cross section is located between two curved portions.
The invention is also a transmission device comprising waveguide elements, at least one bent element of which includes a change in cross section of the waveguide at an end of a curved part.